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SMT Processing and gasketing

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SMT Processing and gasketing | 11 November, 2008

Occasionally solder mask thickness is believed to be the cause of gasketing challenges during paste printing. We currently specific IPC-SM-840 on our fab drawings but this provides no limits or guidelines on thickness.

I do not want to cause an unrealistic spec onto our PCB vendors but am at a loss on what to do for DFM Guidelines, which I am currently revising, and sustaining controllable processes.

I need help from the usual suspects (Dave) on how to fix it.

Thanks, BR

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SMT Processing and gasketing | 11 November, 2008

SM-840, is a material qualification document only, with all the test requirements applicable only to a defined test coupon, not to production boards. So, it's useless for what you're trying to accomplish.

As assemblers use A-610 - Acceptability Of Electronic Assemblies, fabricators use A-600 - Acceptability of Printed Boards. A-600 is the language that you should use in conversing with your fabs. Luckily for you, we have verbage from an outdated version [A-600G] on the very topic, solder mask thickness, that you are in flux [so to speak] about. 3.3.11 Solder Resist Thickness: * Target condition, Class 1, 2, 3: Thickness as specified on procurement documentation. * Accpetable, Class 1, 2, 3: Specified: The solder resist thickness meets the thickness requirements on the procurement documentation [cannot be visually assessed]. * Nonconforming, Class 1, 2, 3: Defects do not meet the above critera.

Now, being from the middle of the back-end of the pack of the SMTneters from an astuteness stand-point, even we sense that's not the answer you were hoping to find. So, we checked IPC-6012 - Qualification & Performance For Rigid Printed Boards, just in case. Here's what we got for you ... 3.8.3 Solder Resist Thickness. Solder resist thickness is not measured unless specified in the procurement documents. If a thickness measurement is required, instrumental methods may be used or assessment may be made using a microsection of the parallel conductors on the E coupon.

So, we guess that we are going to be able to find you running the wuchacallit machine sectioning a bunch of boards from each supplier and using the two that you like as the basis for your 'customer requirement.' Actually, instumental methods [mentioned in 6012] will get you to the answer faster and cheaper. Just rent one of those eddy-current wuchacallit instruments. Wouldn't it be cool if you could sucker your fabs into doing this for you?

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SMT Processing and gasketing | 12 November, 2008

Thanks Dave! I'm fighting the very same battle. Specifications are sparse on the subject, but you offer a very viable solution. (As usual!)

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SMT Processing and gasketing | 12 November, 2008

It's amusing that usually we are complaining to our suppliers about not getting enough of something, here we're complaining about getting too much.

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